1. Field of the Invention
The present invention relates to a heavy duty pneumatic radial tire for use in a truck, a bus and the like, in which the durability of a bead is enhanced while reducing a weight by improving the structure of a winding-up portion of a carcass.
2. Description of the Related Art
As shown in FIG. 8, a carcass has been recently provided with a main unit disposed between bead cores b, b and a ply folding portion a wound around the bead core. Such a ply folding portion a consists of a main portion, which is folded along an inside surface in a tire axial direction, a lower surface in a radial direction and an outside surface in the tire axial direction of the bead core b, and a winding-up portion a1 extending apart from the bead core b. The winding-up portion a1 extends upward from an upper surface bs in the radial direction of the bead core b, and it has been conventionally held between the bead core b and a bead apex rubber c (hereinafter, this bead structure may be referred to as a bead wind structure). Incidentally, the above-described bead structure is disclosed in, for example, Japanese published patent application NOS. H 11-321244 and 2000-219016.
Since the winding-up portion a1 breaking off around the bead core b, that is, not wound around a part of the bead core b in the bead wind structure, a stress at the time of deformation of a tire is not exerted on the winding-up portion a1, thereby effectively suppressing a damage such as a cord looseness starting from the winding-up portion a1. Furthermore, the winding-up portion a1 is short, thus obtaining an advantage of weight reduction of a tire.
On the other hand, although a falling out phenomenon of a carcass ply can be prevented by lockingly holding the winding-up portion a1 between the bead core b and the bead apex rubber c, a locking, or engaging function may become insufficient in the structure per se, since the winding-up portion a1 is relatively short.
This is also, because strong folding-back (that is, so-called spring-back) occurs at the winding-up portion a1, for example, in a green tire molding process since the winding-up portion a1 is short and a folding degree is large. As a result, there may be liable to arise problems of deficient molding due to air remainder such as generation of a cavity between the winding-up portion a1 and the bead core b.
In view of this, the present inventor has devised that a low filled rubber has been previously interposed between the winding-up portion a1 and the bead core b, so that the spring-back is suppressed and the separation of the winding-up portion a1 from the bead core b is decreased by reducing the folding degree.
However, if the winding-up portion a1 is too separated from the bead core b even when the filled rubber is used, the stress exerted on the tip of the winding-up portion a1 becomes large, not to suppress a damage and not to increase the locking force, thereby easily inducing falling up, or loosing. In contrast, if the winding-up portion a1 is not so separated from the bead core b, an impact exerted on the tip becomes large, thereby easily inducing a damage. Moreover, the folding-back remains strong, thereby making it difficult to sufficiently suppress the deficient molding. As a consequence, the problems experienced by the tire in the prior art cannot be solved.
Additionally, as soon as the carcass ply becomes easy to be moved in the falling out direction since the locking force to the winding-up portion a1 becomes weakened owing to the separation, the carcass ply is liable to slide on the bead core b at an inner end position Q in the tire axial direction of the bead core b. Furthermore, it has been found that a damage is likely to occur at the position Q, since a topping rubber of the carcass cord is liable to be split in combination with a strong press-contact force with the bead core b.
Japanese published patent application No. H 08-40026 discloses that a cord layer e is attached to the outer surface of a winding-up portion a1, thereby achieving reinforcement and reducing spring-back, as shown in FIG. 9. However, the cord layer e disclosed in the above-described gazette is inclined in a direction in which a diameter is reduced inward in a tire axial direction. As a consequence, creases or wrinkles are liable to be generated at the end of the winding-up portion a1, thereby easily inducing generation of air remainder, as shown in FIG. 10.